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The Oxford Magazine;

For JULY, 1770.


The wonderful Porofity of Bodies. From Father Feyjoo's Cartas eruditas y curiofas.

The reafon of water admitting more

Conceived my worthy friend to have
ble at a conjecture of mine, that fome-
times the greater levity of water may
depend on having a greater mixture of
air. You fay you cannot conceive water
having a greater or lefs mixture of air,
water being an homogenous and fluid
body, its parts, from the greatest to the
leaft, being every where in fuch imme-
diate contact as to leave no interftice or
room for the admiffion of air.

This, dear Sir, is very far from being the cafe, the mixture or inclufion of air in water is evidently fhewn by the air-pump, where, any quantity of water being put, as the air contained within the machine is extracted, the water, with a kind of ebullition, throws up to the furface, in fmall bubbles, the air which it included. The caufe of this is, the ceffation of the preffure of the outward air, which before, bearing on the water, obftructed the expanfion of the air within; and by this operation all, or the far greater part, of the air, contained in the water is exhaufted, as appears from the ebullition ceafing at the conclufion of the experiment. But, fo difpofed is the water for the reception of new air, that, being again expofed, it admits a quantity of that element equal to what it had before, and even a greater quantity, if heated by the fire immediately before expofing it. This is manifefted by a repetition of the pneumatic experiment.

more open, and its particles more eafily divided..

But there is no need of recurring to the air-pump, for the investigation of this truth. Only expofe water in a glafs veffel, during a frofty night, you will, when frozen, fee in it fome small opake veficles, or whitish spaces. Did thefe spaces, as all the reft, contain only frozen water, there would be no more colour nor opakeness in them than in the reft. What then is their contents? Some aggregated portions of air; for the water being more compreffed by the cold, feveral particles of air, difperfed in it, formed those spaces vacant; fo as, by virtue of its elafticity, to acquire a greater extenfion than before; and this accounts for water, when frozen, occupying a greater fpace, than when in its natural state of liquidity. This, however, is to be underftood of the whole volume, compounded of air and water; for, ftrictly fpeaking, the frozen water alone does in reality take up lefs space than before. Here my friend may wonder how those bubbles, containing only air, which is more tranfparent than water, fhould appear opake: but it is a conftant law in dioptrics, that light is lefs tranfmitted through two mediums of an unequal transparency, than through only a fingle medium, though lefs tranfparent than either. For a conviction



The wonderful Porofity of Bodies.

of this, only look on a piece of coarfe adventitious, and not the proper sub. glafs, and you'll find in it many fmall ftance of the timber. opake fpaces, which are owing only to fome portion of air intercepted in them, at the time of making. If this be not fatisfactory, break a piece of fuch glafs into feveral bits, and in thofe opake spaces you'll find it to be hollow.

You now fee, Sir, that all the particles of water are n t every where in mutual contact, as you imagined, and on which you grounded your denial of air being contained in it; but you have fomething farther to fee, and I now call your attention to what will appear a fhocking paradox, and yet will force your belief. So far are the particles of water from leaving no void interftice between them, I aver, that the void spaces between them are fuch as to take up above eighteen times a greater space than the water itself; fo that, in a veffel which, to outward appearance, feems full of water, the water does not occupy fo much as the eighteenth part of its capacity; the void fpaces intercepted in the water itfelf form a volume above eighteen times larger than that of the very fubftance of the liquor. Here you ask me with what eyes I faw thofe fpaces?-With the eyes of reafon; and thus I prove it.

All philofophers agree, and, what is more, it is demonftrated, that the weight of bodies is proportionate to their density. The molt denfe body is moft heavy, and the most rare is leaft fo; and they more and the lefs precifely conform to the degrees of denfity; that is, a body twice as denfe as another is twice as heavy; if at four times the denfity, four times the fpecific weight. What do you mean by one body's being more denfe than another? The containing more of its proper matter in an equal volume. For inftance, a piece of oak is three times as heavy as another of fir of equal bignefs, becaufe it is three times as denie, that is, it contains within equal dimenfions three times more of its proper fubftance than that of fir; or, which amounts to the fame, the latter is thrice as porous or has three times the void interftices as the former; for what fills the pores in the wood, be it air or any thing else, is

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Farther, gold is known to be nineteen times fpecifically heavier than water, and the excess of denfity being as that of weight, gold contains in an equal volume nineteen times more of proper fubftance than water; fo that in a cubic foot of gold nineteen parts are of the natural fubftance, whereas in a like volume of water there is only one. Water, on the other hand, is fo rare, that in a cubic foot, its proper fubstance, even at moft, takes up only a nineteenth part, all the remainder being occupied by the air or fubtile matter within the numberless void pores or interftices in the water.

A nineteenth part did I fay? not fo much, or rather much lefs. Were gold fo denfe as to exclude all porofity, a comparison of its weight with that of water would only prove that the latter fills the nineteenth part of the fpace, and no more. But gold, being likewife not without pores, and confequently its proper fubftance not taking up the whole pace, i. e. of the cubic foot, the refult of comparing the weight is, that the porofity of the water is greater than eighteen parts of the space. For instance, if the fubftance of the gold, by reafon of its porofity, occupies only two parts of the spaces, leaving the third for the air or fubtle matter within its pores, then is the porofity of water one third more than what takes up eighteen parts of the fpace, confequently the matter contained within its pores will occupy twenty-fix parts of the space, and the proper fubftance of the water only a twenty-feventh part.

Gold porous! will fome fay; yes, doubtlefs; and fo porous, that the point of the fineft needle cannot indicate any part of a piece of gold free from porofity. A proof of this is the diffolution of gold by aqua regia, for how could that liquor diffolve gold but by pervading its pores? Another proof of this porofity is to be met with in Oranam's Recreations Mathematiques, tom. 3. One end of an ingot of gold being put into mercury, it penetrates the other fide, the mercury meeting every where with voids or ducts through

The wonderful Porofity of Bodies.

which it makes its way. A third experiment the Republique des Lettres affords, in what volume I have forgot, the cavity of a golden globe being filled with water, folder it up perfectly fo as not to leave the leaft vent, then prefs it with an engine, and the water exfudes through the pores,

What M. Saurin, of the Royal Academy of Sciences, fays concerning the porofity of gold, will, to a vulgar philofopher, appear chimerical: his words are thefe; I take upon me to affirm, paradoxical as it may feem, that though in maintaining there is not in a piece of gold a hundred millionth part of the real fubftance of gold, no positive proof can be adduced of fuch aftonishing porofity, yet may all the philofophers living be fafely challenged to prove the contrary." Memoirs, 1709. p. 143.

I fubfcribe to M. Saurin's affertion, and with this addition, that if the propofition admits of no pofitive proof, yet its not being matter, being fuppofed impoffible, may be evidenced by a problem that is infinitely divifible. The great Newton has demonstrated it: "Any particle of matter, however minute, and any finite space how great foever, being given, the matter of that particle may diffuse itself through, and fill all that space, that there fhall not be in it any one pore, the diameter of which exceeds any given line how fmall foever:" The demonftration of this problem proceeding on the fuppofition of the divifibility of matter ad infinitum, a little reflection will clear it up, and arther fhew, as an evident inference from it, the poffibility of the real fubftance of the gold not occupying in reality even the hundredth millionth part of the space which it occupies in ap


So much for the poffibility; as to the reality, a method has occurred to me for proving the porosity even of the most dense bodies to be innumerably greater than is commonly imagined. Glafs is a denfe body, yet fo porous, that to say the matter contained in its pores fills a hundred thoufand, yes, ive hundred thousand times more fpace than its own fubftance is not beyond uth. According to philofophers it is rough direct pores that light is tranf


mitted; yet its rays far from falling on
it, only perpendicularly ftrike it in
every point of obliquity, and herein
confifts the tranfparency of glass, and
this is an evidence of aftonishing po-
rofity. Let us fuppofe light in a per-
pendicular incidence on glafs, to trans-
mit only a tenth part of its rays; ac-
cording to this computation, light, in
a perpendicular direction, meets with
ftrait pores, occupying the tenth part
of the space where the glafs is. Light
unquestionably may ftrike glafs ob-
liquely in more than ten millions of
different directions; that is, in all posli-
ble angles of incidence, granting an ob-
lique direction tranfmits not fo many
rays as a perpendicular; and farther,
that from thence it follows, that in an
oblique direction they meet not so many
pores as in a perpendicular; or rather
that they are the fewer, the more ob-
lique the direction, or the greater the
inclination. Computing both the greater
and lefs inclinations, let us, on an ave-
rage, allow it in every direction to
meet with no more straight pores than
what will fill up the twentieth part of
the space. The perpendicular direction
is not taken into account; for being only
one, its few pores would stand us in no
great ftead. The estimate for the oh-
lique direction is, that the straight pores
of the glass, or the matter contained in
them, takes up five hundred thousand
times more space than the very matter
of the glafs. And may not innumer-
able other pores be fuppofed in that not
ftraight thro' the whole thickness? why
not? and efpecially speaking of those
which cut it obliquely? I rather incline
to think, that if lefs light be transmitted
by an oblique incidence of the rays,
when the inclination is any thing great,
it is that though it meets with as many
ftraight pores as in a perpendicular inci-
dence; that direction does not run
through all the space which the light is
to pafs in that incidence, thofe minute
ducts fuffering fome inflection, fraction,
or obftruction. And on this account,
as is faid above, it did not follow from
the tranfmiffion of light through an
oblique incidence, that the rays met
with fewer pores than in a perpendi
cular incidence.

From the preceding eftimate may be


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